1. Field of the Invention
The present invention relates to a diffuse and a non-specularly reflective polarizer device, particularly well suited for liquid crystal displays. More particularly, the present invention relates to a polarizer device with a plurality of thin, elongated elements for (i) transmitting light having a first polarization orientation perpendicular to the elements, and (ii) reflecting light having a second polarization orientation parallel with the elements, and wherein the elongated elements each have an exposed surface which together define a textured surface for reflecting the second polarization in a diffuse manner, or in a non-specular manner with respect to a reference plane.
2. Prior Art
A typical liquid crystal display device comprises a layer of liquid crystal material sandwiched between front and back transparent plates. Transparent electrodes are located on the inner surfaces of the transparent plates and used to apply electrical signals which alter the light transmission properties of the liquid crystal layer. The transparent electrodes are typically patterned to define the pixel structure of the display device.
The preferred and most commonly used form of liquid crystal display utilizes the well-known "twisted nematic" liquid crystal effect. The twisted nematic effect is preferred because it offers excellent contrast ratio, low driving voltages and a sharp response threshold compatible with current drive circuit technology, wide viewing angle, and good gray-scale rendition.
In a twisted nematic liquid crystal display, the display sandwich also includes linear polarizers affixed to the outer surfaces of the front and back transparent plates and having orthogonal axes of polarization. The liquid crystal layer is designed such that the polarization vector of light transmitted through the layer is rotated 90 degrees in the absence of an applied electric field, but not rotated in the presence of an electric field. Thus, in the absence of an electric field, the light transmitted through one polarizer is reoriented to pass through the opposing polarizer such that the panel is transparent and appears bright to the observer. In the presence of an electric field, the light transmitted by one polarizer is not rotated and is thus blocked by the second polarizer. Thus the panel is opaque and appears dark to the viewer. In this manner, the transparent electrodes can be used to apply electric fields to selected areas of the panel to create a visible image in the form of light and dark pixels.
In many applications, the liquid crystal display device is illuminated by a light source located behind the rear side of the sandwich and viewed from the opposing side. In this case the visible image is created by light that passes through the panel a single time. However, in some applications, such as portable communications equipment, low power consumption is critical and the display is illuminated primarily by ambient light. In this case, a reflective element is located behind the liquid crystal sandwich such that the ambient light passes through the sandwich, reflects from the reflective element, and passes again through the sandwich in the opposing direction to the viewer. Thus the image seen by the viewer is formed by light which has passed through the liquid crystal device twice.
The problems with current ambient-illuminated twisted nematic liquid crystal devices relate to the fact that the light passes through the device twice. The most significant problem, normally referred to as "parallax", occurs because the reflector is located behind the rear transparent plate and the rear linear polarizer at a considerable distance from the liquid crystal layer. The ambient light entering the display is spatially modulated by the liquid crystal layer to form a pattern of light and dark areas where the light impinges upon the rear reflector. After reflection, the light passes through the liquid crystal device in the reverse direction and is again spatially modulated. However, since the display is normally illuminated and viewed at oblique angles with respect to the display surface, the images formed by the two passes through the liquid sandwich generally do not overlap, and a double image, or shadow image, is seen by the viewer under most conditions. Although the shadow image is currently accepted for low resolution displays such as those used in portable phones and calculators, this phenomenon does limit the resolution, or minimum pixel size, of ambient illuminated twisted nematic liquid crystal displays, and prevents their application in products which require high information-density displays, such as lap-top computers.
An additional problem with current ambient illuminated twisted nematic liquid crystal displays is the additional loss of brightness that occurs due to absorption in the linear polarizers. Note that this would not be a problem with theoretical polarizers that transmit 100% of one polarization while absorbing 100% of the orthogonal polarization. However, current linear polarizers only transmit 90% or less of the preferred polarization. The additional absorption during the second pass through the liquid crystal sandwich results in a loss of at least 20% of the possible display brightness.
Alternate methods have been proposed to eliminate the parallax problem in ambient illuminated twisted nematic liquid crystal displays. One method, as described in U.S. Pat. Nos. 4,492,432 and 5,139,340, is to utilize an alternate liquid crystal electro-optical effect that only requires a polarizer on the front side of the display. Since the rear polarizer is not required, the rear reflector can be located on the inner surface of the rear transparent plate in immediate proximity to the liquid crystal layer. While this method eliminates the parallax problem, displays using this method do not provide the high contrast, wide viewing angle, fast response, and smooth gray scale rendition provided by twisted nematic liquid crystal display devices.
Still another method is the Polymer Dispersed Liquid Crystal Display (PDLC) in which the liquid crystal layer itself functions as a diffuse reflector, eliminating the need for polarizers or a separate reflector. While this method offers the potential for high display brightness, the PDLC requires high drive voltages and complex drive waveforms that are not compatible with current drive circuit technology. Given these problems with alternative technologies, it would be an advancement in the art to develop a display technology which retains the advantages of the twisted nematic liquid crystal technology while eliminating the parallax problem.
U.S. Pat. No. 4,688,897, issued to Grinberg, proposes to improve ambient illuminated twisted nematic liquid crystal displays by incorporating a wire grid reflective polarizer within the twisted nematic liquid crystal device. The wire grid functions as the rear polarizer, as a specular reflector, and as the rear electrical contact to the liquid crystal layer. While this approach does is eliminate the parallax by virtue of having the rear reflector in intimate contact with the liquid crystal layer, it does so by sacrificing many of the other attractive features of the twisted nematic display by using a specular, rather than a diffuse, reflector.
It has long been recognized that a specular reflector is not acceptable in ambient illuminated liquid crystal displays for three reasons. First, the specularly reflected display image must be viewed along the same axis as the specular reflections that occur naturally from the front surface and internal surfaces of the display sandwich. These surface reflections (which are not spatially modulated to form an image) may be 5% or more of the incident light. Because of the absorption of the polarizing elements, the maximum brightness of the display image cannot be more than 40% of the incident light. Thus the maximum possible contrast ratio for the display is 40%/5% or 8:1, and may be much less. Second, the viewing angle and brightness of a display with a specular reflector are determined strictly by the illumination source. For example, while the performance of the display might be acceptable under diffuse interior illumination, the viewing angle would be extremely small when illuminated by a point source, and the brightness of the display would be beyond the capacity of the human visual system when viewed under direct sunlight. Third, displays with specular reflectors commonly have objectionable color fringes caused by interference between the multiple reflections within the device. For these reasons, the above proposal, while moving the polarizer and reflector into the liquid crystal cell to eliminate the problem of parallax, does not provide an improved ambient-illuminated liquid crystal display device. Thus, there still exists a need for a substantially improved ambient-illuminated display that overcomes the parallax problem while maintaining the performance advantages of the twisted nematic liquid crystal effect.
Therefore, it would be advantageous to develop a liquid crystal display device with reduced parallax, or a polarizer device capable of reducing or eliminating parallax. It would also be advantageous to develop a polarizer device capable of diffusely reflecting light. It would also be advantageous to develop a polarizer device capable of reflecting one polarization in a nonspecular manner.